If you go to the container forum you'll find some answers, I know pumice has come up recently, but the search isn't working again at the moment.

Here is a link that might be useful: Container forum

Mel teaches SFG in other countries with no peat or vermiculite. He recommends diverse compost exclusively in that case. No need to buy expensive amendments. Good luck and enjoy the experience!

You don't need vermiculite at all. My own sqft garden is purely compost, some topsoil/garden soil and a bit of sand. It grows vegetables beautifully! Mel's mix has its merits, but it isn't totally necessary for the rest of the program to work.

Ditto that. My last 4x4 I built was for sweet potatoes exclusively. I used straight compost. Store bought 8 bags. One was black hen. I think for this instance Mel's mix is overkill.